In the past it has been proposed to use a metal layer for the gate electrode in a transistor with which it is possible to obtain a high ON current. However the following problems arise if the gate electrode is configured using only a metal layer.
(1) Because control of the threshold voltage using the gate electrode depends on the film thickness, it is difficult using only a metal layer to achieve a thickness that can satisfy both the need for a desired threshold and a low resistance.
(2) From the point of view of ease of machining, it is problematic to arrange that the entire gate electrode comprises a metal layer.
There have thus been proposed transistors having a gate electrode employing a metal gate construction in which a silicon layer is laminated onto a metal layer.
Further, by employing this gate electrode, having a metal layer and a silicon layer, in combination with a gate insulating film having a high-dielectric constant insulating layer (high-K film) that is higher than that of a silicon oxide film, leakage currents which pass through the gate insulating film as a result of a quantum tunneling effect can be reduced, and the transistor can be effectively miniaturized and more highly integrated.
Non-patent literature article 1 (Extended Abstracts of the 2007 International Conference on Solid State Devices and Materials, Tsukuba, 2007, pp. 16-17, B. P. Linder et al.) discloses a transistor equipped with a gate insulating film comprising a high-dielectric constant insulating layer and a gate electrode comprising a metal layer.